[Pafgbt] save PAF cross-correlations rather than formed beam outputs

[Rick Fisher]

Brian,

Quite right.  You'd need to anticipate the spectral resolution needed for 
any possble application of the data and provide the data storage/bandwidth 
that goes with it.  We should see what's typical for pulsar searches.

Rick

On Mon, 20 May 2013, Brian Jeffs wrote:

> Rick,
> I agree that you have much more flexibility to try different beamformer
> designs, detection algorithms, interference mitigation techniques,
> superresolution, calibration correction, etc. if you store and operate on
> the accumulated cross products (correlation matrices).  However, you give up
> the ability to do fine resolution spectral processing.  You are stuck with
> the coarseness of the correlator's frequency channelization.  I don't know
> how problematic this is for some applications, such as pulsar searches,
> where fine spectral resolution may be needed.
> 
> Brian
> 
> On May 20, 2013, at 6:38 AM, Anish Roshi wrote:
> 
> 
> Yes indeed. We can form images with beams with different optimization
> if the correlations are recorded.
> Anish
> 
> 
> On Sun, May 19, 2013 at 9:57 AM, Rick Fisher <rfisher at nrao.edu> wrote:
>       Brian, Karl,
>
>       In trying to understand the ASKAP data processing
>       architecture, I'm
>       beginning to understand the fundamental importance of
>       saving the
>       cross-products between array element outputs in our own
>       PAF data
>       processing.  In forming beams you throw away a lot of
>       information in the
>       array's field of view that can be recovered only by
>       forming many beams
>       with very close spacing (much closer than HPBW/2).  This
>       has important
>       consequences for the sensitivity to point sources, as in
>       the search for
>       pulsars.  Hence, I would suggest that the most important
>       archived outputs
>       from your signal processor are the cross-products rather
>       than formed
>       beams.  For a given data storage volume, there's more
>       information in the
>       cross-products than in the formed beam outputs.  In some
>       respects, the
>       "beam" concept is a holdover from a waveguide feed where
>       there's only one
>       output, and most of the information in the focal plane is
>       reflected back
>       into the sky.
>
>       Rick
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